Sulphate control in chloride leaching processes

Abstract

The deportment of sulphate during the ferric chloride leaching of a pyritic ZnPbCuAg bulk concentrate has been elucidated, and several sulphate control options have been investigated. Sulphate accumulates in the pregnant leach solution even in the presence of high concentrations of dissolved lead. The addition of > 0.6 M CaCl 2 to the leaching medium precipitates anhydrous CaSO 4, and gives final sulphate concentrations of 1–3 gl −1SO 4. Calcium sulphate contamination of the intermediate PbCl 2 product is a concern, but this problem can be minimized by rapid crystallization of the PbCl 2. Although sulphate can be limited by the precipitation of potassium jarosite, the final SO 4 concentration of the leach liquor is high and all the free acidity must be neutralized prior to jarosite precipitation. Significant contamination of the PbCl 2 product by KPb 2Cl 3 also occurs. Sodium jarosite and ammonium jarosite are not precipitated under the conditions examined. Barium chloride additions effectively control sulphate, and the BaSO 4 precipitation reaction is nearly stoichiometric. Excess BaCl 2, however, results in the precipitation of BaCl 2-H 2O which impedes residue filtration and contaminates the PbCl 2 intermediate product.